The removal of sulfur contaminants, specifically mercaptans, from hydrocarbon streams using caustic is well known. Likewise, the oxidation of these mercaptans to disulfides by contacting the rich caustic stream with a solid catalyst in the presence of oxygen followed by separation of the disulfides from the treated caustic is also well known. Regardless of the oxidation and/or separation processes employed, there will always be residual sulfur compounds remaining in the treated caustic solution. With pressing needs for more economical processes that also are more compact, there is a need to replace traditional solvent washing with a smaller more efficient polishing process to yield sulfur free caustic that can be reused to treat sulfur contaminated hydrocarbons.
The enactment of the US Clean Air Act of 1990 has reached its zenith in North America with the gasoline pool being required to contain less than 10-wppm of sulfur. This means from a practical standpoint that the refinery normally makes a gasoline pool containing less than 5-wppm to allow for pipeline contamination of residue wall “clingage” sulfur from previous shipments and the accuracy of the testing method dictated by the Clean Air Act.
Another consequence of the Clean Air Act of 1990 has been the shutting down of the small inefficient refiners in America going from 330 plus refiners in 1980 to less than 175 refiners in 2007. No new refiners have been built in the past 25 years, but refiner expansions and imports have satisfied the gasoline demand in America.
The existing refiners have also gone to higher severity Fluid Catalytic Cracking Unit operations to reduce the amount of burner fuel while producing additional higher octane gasoline and increased olefin production. These olefins are propane/propylene and butane/isobutane/isobutylene. These are the feedstocks for the next processing step which is an alkylation unit. Some refiners alkylate amylenes (pentene) depending on their economic models.
Most refineries use either an HF (hydrofluoric acid) or a sulfuric acid alkylation unit to alkylate mixed butylenes or mixed propylene's and butylenes. Alkylation is a process where isobutane reacts with the olefin to make a branched chain paraffin. Since sulfur is detrimental to the alkylation process, a caustic treating system is in place at most refineries to extract the easily extracted methyl and ethyl mercaptans and the more difficult propyl mercaptans present in the mixed olefinic liquid petroleum gas (“LPG”) stream.
Typically, liquid-liquid contactors are employed for the caustic treatment and in some cases fiber-film contactors as described in U.S. Pat. Nos. 3,758,404; 3,977,829 and 3,992,156, all of which are incorporated herein by reference. To conserve caustic, a caustic regenerator is almost always employed. A typical process flow scheme for treating LPG involves a first caustic treatment using at least one liquid-liquid contactor to extract the sulfur contaminants, typically mercaptans, from the LPG feed, which generates a “spent” caustic solution that is rich in mercaptan or so called rich caustic, separating the LPG in the contactor, oxidizing the rich caustic to convert mercaptans to disulfides (typically referred to as disulfide oil (“DSO”)) which generates an “oxidized” caustic solution, and then using a gravity separator to separate the DSO from the oxidized caustic solution. In some instances a granular coal bed is used in conjunction with the gravity settling device as a coalescer to further assist in the separation of the DSO from the oxidized caustic. Once the DSO is removed, the regenerated caustic can be further processed and then recycled, where it is mixed with fresh make-up caustic and used in the liquid-liquid contactors to treat the LPG feed. More typically, a further polishing processing is required in order to reduce the unconverted mercaptans and residual DSO to preferably below 5 weight ppm as sulfur. The presence of substantial mercaptans in regenerated caustic is undesirable because it can cause a loss of extraction efficiency and presents a potential for downstream formation of disulfides. The presence of substantial DSO in regenerated caustic leads to undesirable re-entry or back extraction of DSO into hydrocarbon during the hydrocarbon-caustic extraction process.
Solvent washing is a known technology and is often used as a polishing step to extract residual DSO from caustic. However, due to mass transfer and equilibrium limitations, these solvent washing unit operations usually require multiple stages with higher capital and operating costs. Besides, solvent washing is ineffective to remove mercaptans from caustic. Similarly, centrifugal process and membrane separation suffer from high costs and inability to achieve less than 5 weight ppm sulfur.
Adsorptive polishing is another technology that can be used. Adsorptive desulfurization has been applied to remove sulfur compounds from hydrocarbons such as gasoline and diesel. Examples are shown in U.S. Pat. Nos. 7,093,433; 7,148,389; 7,063,732; and 5,935,422. However, the adsorbents reported in these patents and in other literature are ineffective in caustic media.
Therefore, there remains a need to develop a technology that can economically removes both disulfides and mercaptans from caustic as a polishing process to achieve less than 20 weight ppm sulfur, preferably less than 5 ppm and most preferably less than 2 ppm.
My process uses single step oxidation and adsorption separation (OAS) to remove both disulfides and mercaptans from caustic solution. The OAS process replaces solvent washing as a polishing step and, when used after bulk DSO separation, converts residual mercaptans to DSO and removes all residual DSO including the DSO that was formed in-situ from mercaptans. Further, my process is extremely economical compared to traditional methods for removing residual sulfur compounds from caustic solutions by minimizing both capital and operating costs. These and other advantages will become evident from the following more detailed description of the invention.